Autocatalytic oxidations of primary hydroxyl groups of cellulose in phosphoric acid with halogen oxides

Selective oxidation of the primary hydroxyl groups in cellulose has been achieved in phosphoric acid with: (i) sodium bromate, (ii) sodium chlorate, and (iii) sodium chlorite as stoichiometric oxidants in the presence of catalytic amounts of sodium bromide and chloride. With these reaction systems, where the oxidising oxyhalogen species are formed in situ, the primary alcohol groups could be completely (>90%) oxidised to carboxylic acids. Ring cleavage also occurred, especially with chlorate. The carbonyl groups formed by oxidation of secondary hydroxyl groups were subsequently reduced with sodium borohydride. This oxidation-reduction sequence of secondary alcohol groups gave epimerisation. Polymer degradation differed markedly. Using water soluble pullulan with ?Mw?~170 kg/mol as a probe for degradation, bromate yielded a polymer with ?Mw?~70 kg/mol while chlorate and chlorite yielded highly degraded polyuronates of ?Mw?~10 kg/mol. A common mechanism for the oxidation of primary hydroxyls is suggested to account for the selectivity observed.